1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device such as an electrical erasable and programmable read only memory (EEPROM), and more particularly, to a nonvolatile semiconductor memory device capable of converging a threshold voltage after an erase or write operation is carried out.
2. Description of the Related Art
For example, an EEPROM cell includes a P-type semiconductor substrate having an N.sup.+ -type source region and an N.sup.+ -type drain region, a floating gate over the semiconductor substrate between the source region and the drain region, and a control gate over the floating gate. In an erase mode, a positive voltage is applied to the drain region, and the control gate is grounded. As a result, electrons stored in the floating gate are expelled therefrom to the drain region utilizing Fowler-Nordheim tunneling. To realize the Fowler-Nordheim tunneling, an insulating layer between the floating gate and the semiconductor substrate is made very thin, for example, about 6 to 20 nm.
In the above-described erase mode, however, when more or fewer electrons than those injected in a write mode are expelled from the floating gate in accordance with the thickness of the insulating layer and the like, and as a result, the threshold voltage of the EEPROM cell is greatly fluctuated. At worst, the threshold voltage of the EEPROM cell is made negative by an over-erase operation, i.e., the EEPROM cell becomes a depletion-type transistor. In this case, when writing data into this EEPROM cell, a large current flows between the source region and the drain region to reduce the potential at the drain, so that it is impossible to carry out a write operation. Also, currents flow through non-selected EEPROM cells.
In order to converge the threshold voltage of the EEPROM cell after the erase operation thereupon is completed, a threshold voltage converging method is carried out (see: S. Yamada et al., "A SELF-CONVERGENCE ERASING SCHEME FOR A SIMPLE STACKED GATE FLASH EEPROM", IEDM Tech. Dig., pp. 307-310, 1991). This will be explained later in detail.
In the above-described prior art threshold voltage converging method, however, since use is made of an avalanche breakdown occurring in the vicinity of the drain region, a high voltage such as 5 V has to be applied to the drain region. As a result, a large capacity of pump (step-up) circuit for generating such a high voltage is required, thus reducing the integration.